1. Field of the Invention
The present invention relates to an image forming apparatus employing electrophotography, such as a printer or a copying machine.
2. Description of the Related Art
In an image forming apparatus employing electrophotography, a laser beam corresponding to image information is applied to a uniformly charged photosensitive member, thereby forming an electrostatic latent image. Then, developer is supplied to the electrostatic latent image by a developing unit to make the image visible. Further, the image is transferred from the photosensitive member to a recording material, thereby forming an image on the recording material. A contact charging method using a charging device for the photosensitive member is widely adopted. According to the contact charging method, a charging roller is brought into contact with the surface of the photosensitive member, and voltage is applied to the charging roller to thereby charge the surface of the photosensitive member.
The charging bias voltage applied to the charging roller may be solely a direct current voltage. However, in many cases, a bias voltage is employed which is obtained by superimposing an alternating current voltage exhibiting a peak-to-peak voltage (Vpp) equal to or greater than double the discharge start voltage at the start of the application of the direct current voltage on a direct current voltage Vdc corresponding to a desired dark portion potential Vd on a photosensitive member. This charging method is superior in uniformly charging the surface of the photosensitive member, and helps to clear away local unevenness in potential on the photosensitive member.
When the peak-to-peak voltage is low, faulty charging may be generated. On the other hand, when the peak-to-peak voltage is too high, the roughness on the surface of the photosensitive member may increase, thus resulting in generation of longitudinal streaks on the image. Thus, it is desirable to apply a requisite minimum peak-to-peak voltage high enough not to generate faulty charging. Japanese Patent No. 3902974 discusses a superior method which helps to achieve compatibility between a reduction in power circuit cost and the application of a more suitable charging bias. In this method, a plurality of alternating current voltages differing in peak-to-peak voltage is applied to the photosensitive member, and each alternating current flowing through the photosensitive member is detected, then determining the charging bias at the time of image formation according to a peak-to-peak voltage leading to a minimum electric current value equal to or greater than a reference current. Further, as discussed in Japanese Patent No. 3903019, there is a method according to which the control to determine the charging bias at the time of image formation is conducted during an initial rotation when the power source is turned on. The initial rotation refers to performing an image formation preparing operation while rotating the photosensitive member after turning the power source on. As a result, there is no need to control operation during image formation, so that it is possible to make the period of time elapsing until the image output so much the shorter.
However, in the case of the method as discussed in Japanese Patent No. 3903019, in which the charging bias at the time of image formation is determined through the initial rotation at the time of turning-on of the power source and in which there is a difference in the driving speed of the photosensitive member between the time of initial rotation when the power source is turned on and the time of image formation, the following problem is involved. By way of example, this will be discussed in relation to the case where the driving speed of the initial rotation at the time of turning-on of the power source is lower than the driving speed at the time of image formation. The problem involved in this case lies in the fact that the charging bias determined at the time of initial rotation (at the time of non-image-formation), when the driving speed is low, cannot be applied as it is to the case where the driving speed is high (at the time of image formation). In the case of the charging system in which a bias obtained through superimposition of an alternating current voltage is applied, the frequency of the alternating current voltage applied is increased according to the driving speed, so that the amount of electric current flowing per unit time is made constant, making it possible to effect the charging in a satisfactory manner. Due to the characteristics of the electric circuit, the alternating current voltage waveform may be somewhat changed as a result of the change in the frequency of the charging bias, which also means a change in the alternating current value. Thus, there is the possibility of the charging bias determined when the driving speed is low not being the proper charging bias in the case where the driving speed is high.
To solve this problem, a method is available according to which the control to determine the charging bias is not performed during the initial rotation but immediately before the image formation. However, this method takes so much the more time, resulting in a delay in image output.